Science and Reason: Global warming could halt ocean circulation

It's looking more and more as though one of the more unwelcome consequences of global warming may come to pass by 2100 -- the paradoxical possibility of much colder termperatures in Europe and the northeastern U. S.

Weakening of ocean's circulation could actually cool things down

Things get worse if the north Atlantic water is swamped by fresh water from rain, continental rivers or melting glaciers, especially from the Greenland ice cap. Fresh water dilutes the salty ocean, making it less dense and even more buoyant. In a worst-case scenario, the north Atlantic water is so warm and buoyant that it can't sink at all, as if it were an inflated balloon floating atop a swimming pool.

When that happens, the oceanic heat-transport system stalls or totally breaks down. Like a blood clot in a vein, the buoyant water blocks the oceanic circulation, preventing new warm tropical water from flowing north, and Europe's average temperature drops by several degrees.

How likely is this to happen? Some researchers think it's a lot more probable than recently believed:

Global warming could halt ocean circulation, with harmful results

Absent any climate policy, scientists have found a 70 percent chance of shutting down the thermohaline circulation in the North Atlantic Ocean over the next 200 years, with a 45 percent probability of this occurring in this century. The likelihood decreases with mitigation, but even the most rigorous immediate climate policy would still leave a 25 percent chance of a thermohaline collapse.

“This is a dangerous, human-induced climate change,” said Michael Schlesinger, a professor of atmospheric sciences at the University of Illinois at Urbana-Champaign. “The shutdown of the thermohaline circulation has been characterized as a high-consequence, low-probability event. Our analysis, including the uncertainties in the problem, indicates it is a high-consequence, high-probability event.”